This is something that has been gradually getting worse but is now at a point where I can't really use the car.

At about 2500rpm in any gear if you try to accelerate the car lacks power and kangeroos slightly but its now at a point where there is very little power and hills are becoming a challenge unless I rev the crap out of the engine.

I pulled over and checked the HT leads and the air filter, all seems fine, does anyone else have any suggestions. I'm going to service the car this weekend (new plugs etc) so I'm hoping that will fix it.

Sounds like ignition coils if the leads and plugs are ok. Always replace the coils in pairs if you can afford it. Approx £25 each.

Check airflowmeter/Maf or whatever its called on these.
Had same problem on a mk3 golf gti. Disconnected it and ran fine till I took it off and cleaned with electrical cleaner and put back on

MAF will sorta just be lacking

If its lumpy / missing, i'd throw as a bare minimum Spark and HT's to see if this cures the fault

But the more load thats put on the engine (hills for example) its worse, that sounds more like a coil pack failure

So beaware spark/ht's may also require a new coil

Tbh if it was me - i would do the same

Ziggy